testing google sheet editor addon fails with error 403

I have a Sheets editor addon. it is a stand alone project connected to a standard google cloud project (as opposed to the default project).

I want to "Publish -> Test as add-on..." from the legacy editor menu.

1. I choose one document owned by same account
2. Set "installed and enabled" on the bottom of the same screen
3. addon is now added to the document Addon menu
4. When launching the addon, there's a prompt for Authorization approval.
5. but at the end I get an Authorization error 403. see attached screen shoot.

This youtube video suggest to add a user in the OAuth consent screen. so I did, but it didn't help.

Please advise.

Creating the OAuth screen is not enough for this - you should also add some Test users for this.

Even though you are the owner of the project, you should still add your address to the "Test users" section if you are the one who is testing the add-on.